The development of technologies to determine the position of a mobile device has enabled application developers and wireless network operators to provide location-based and location-aware services. Examples of these are guiding systems, shopping assistance, friend finder, presence services, community and communication services and other information services that give the mobile user information about his or her surroundings or that use this information to enhance their services.
In addition to the commercial services facilitated by these technologies, location-based emergency services are also being deployed. The governments in several countries have put specific requirements on the network operators to be able to determine the position of an emergency call. For instance, governmental requirements in the United States specify that mobile networks must be able to determine the position of a certain percentage of all emergency calls and further include accuracy requirements. The requirements make no distinctions between indoor and outdoor environments.
In many environments, the position can be accurately estimated by using positioning methods based on Global Navigation Satellite Systems (GNSS), such as the well-known Global Positioning System (GPS). However, GPS-based positioning may often have unsatisfactory performance, especially in urban and/or indoor environments.
Complementary positioning methods may also be provided by a wireless network to augment GPS technology. In addition to mobile terminal-based GNSS (including GPS), the following methods are currently available or will be soon be included in the Long-Term Evolution (LTE) standards developed by the 3rd-Generation Partnership Project (3GPP):                Cell ID (CID),        E-CID, including network-based angle-of-arrival (AoA),        Assisted-GNSS (A-GNSS), including Assisted-GPS (A-GPS), based on satellite signals,        Observed Time Difference of Arrival (OTDOA),        Uplink Time Difference of Arrival (UTDOA)—currently being standardized.        
Several positioning techniques are based on time-difference-of-arrival (TDOA) or time-of-arrival (TOA) measurements. Examples include OTDOA, UTDOA, GNSS, and Assisted-GNSS (A-GNSS). A typical, though not the only, format for the positioning result with these techniques is an ellipsoid point with an uncertainty circle/ellipse/ellipsoid, which is the result of intersection of multiple hyperbolas/hyperbolic arcs (e.g., OTDOA or UTDOA) or circles/arcs (e.g., UTDOA, GNSS, or A-GNSS).
Several techniques, such as Adaptive Enhanced Cell Identity (AECID), may involve a mix of any of the methods above, and are thus regarded as “hybrid” positioning methods. With these methods, the position result can be almost any shape, but in many cases it is likely to be a polygon.
Cellular-based positioning methods (as opposed to satellite-based methods, for example) rely on knowledge of anchor nodes' locations, i.e., the fixed locations from which measured signals are transmitted (e.g., for OTDOA) or the fixed locations at which signals transmitted by mobile devices are measured (e.g., for UTDOA). These fixed locations may correspond, for example, to base station or beacon device locations for OTDOA, Location Measurement Unit (LMU) antenna locations for UTDOA, and base station locations for E-CID. The anchor nodes' locations may also be used to enhance AECID, hybrid positioning, etc.
Positioning Architecture
In 3GPP, location-based services are known as Location Services (LCS). Three key network elements in an LTE positioning architecture are the LCS Client, the LCS target and the LCS Server. The LCS Server is a physical or logical entity that manages positioning for a LCS target device by collecting measurements and other location information, assists the target device in measurements when necessary, and estimating the LCS target location. A LCS Client is a software-based and/or hardware entity that interacts with a LCS Server for the purpose of obtaining location information for one or more LCS targets, i.e., the entities being positioned. LCS Clients may reside in a network node, an external node (i.e., a network external to a cellular network), a Public Safety Access Point (PSAP), a user equipment (or “UE,” in 3GPP terminology for an end-user wireless station), a radio base station (or “eNodeB,” in LTE systems), etc. In some cases, the LCS Client may reside in the LCS target itself. An LCS Client (e.g., an external LCS Client) sends a request to LCS Server (e.g., a positioning node) to obtain location information. The LCS Server processes and services the received requests and sends the positioning result (sometimes including a velocity estimate) to the LCS Client.
In some cases, the position calculation is conducted by a positioning server, such as an Enhanced Serving Mobile Location Center (E-SMLC) or a Secure User-Plane Location (SUPL) Location Platform (SLP) in LTE. In other cases, the position calculation is carried out by the UE. The latter approach is known as the UE-based positioning mode, while the former approach includes both network-based positioning, i.e., position calculation in a network node based on measurements collected from network nodes such as LMUs or eNodeBs, and UE-assisted positioning, where the position calculation in the positioning network node is based on measurements received from UE.
LTE Positioning Protocol (LPP) is a positioning protocol for control plane signaling between a UE and an E-SMLC, which is used by the E-SMLC to provide assistance data to the UE and by the UE for reporting measurements to the E-SMLC. LPP has been designed in such a way that it can also be utilized outside the control plane domain such as in the user plane in the context of SUPL. LPP is currently used for downlink positioning.
LTE Positioning Protocol Annex (LPPa), sometimes referred to as LTE Positioning Protocol A, is a protocol between the eNodeB and the E-SMLC, and is specified only for control-plane positioning procedures, although it still can assist user-plane positioning by querying eNodeBs for information. For example, LPPa can be used to retrieve information such as positioning reference symbol (PRS) configuration in a cell for OTDOA positioning, or UE sounding reference signal (SRS) configuration for UTDOA positioning, and/or eNodeB measurements. LPPa may be used for downlink positioning and uplink positioning.
FIG. 1 illustrates the UTDOA architecture currently under discussion in 3GPP, including nodes found in the Radio Access Network (RAN) and the core network, as well as an external LCS Client. Although uplink (UL) measurements may in principle be performed by any radio network node, such as the illustrated LTE eNodeB 110, the UL positioning architecture also includes specific UL measurement units, known as Location Measurement Units (LMUs), which are logical and/or physical nodes that measure signals transmitted by a target UE, such as the UE 130 illustrated in FIG. 1. Several LMU deployment options are possible. For example, referring to FIG. 1, LMU 120a is integrated into eNodeB 110, while LMU 120b shares some equipment, e.g., at least antennas, with eNodeB 110. LMU 120c, on the other hand, is a standalone physical node comprising its own radio components and antenna(s).
While the UTDOA architecture is not finalized, there will likely be communication protocols established for communications between a LMU and positioning node, and there may be some enhancements to support UL positioning added to the existing LPPa or to similar protocols.
In particular, a new interface between the E-SMLC and LMU is being standardized for uplink positioning. This interface, known as SLm, is terminated between a positioning server, e.g., the E-SMLC 140 pictured in FIG. 1, and an LMU. It is used to transport messages according to the SLmAP protocol, a new protocol being specified for UL positioning, between the E-SMLC and the LMU. SLmAP can be used to provide assistance data to an LMU, as discussed in further detail below. This protocol may also be used by the LMU to report to the E-SMLC results of measurements on radio signals performed by the LMU. The SLmAP protocol was previously referred to as the LMUp protocol; thus it should be understood that references herein to SLmAP are referring to a developing protocol referred to as LMUp elsewhere, and vice versa.
In LTE, UTDOA measurements, known as UL relative time-of-arrival (RTOA) measurements, are performed on Sounding Reference Signals (SRS). To detect an SRS signal, an LMU 120 needs a number of SRS parameters to generate an SRS sequence that is correlated against the received signal. These parameters are not necessarily known to LMU 120. Thus, to allow the LMU to generate the SRS sequence and detect the SRS signals transmitted by a UE, SRS parameters must be provided in the assistance data transmitted by the positioning node to LMU; these assistance data would be provided via SLmAP. The specific contents of the assistance data to be provided to LMUs by a positioning node are currently being discussed. It has been proposed that the same parameters should be signaled from the eNodeB to a positioning node.
TABLE 1Parameter CategoryParametersGeneralC-RNTIServing eNB eCGI, PCIUL-EARFCNCyclic prefix ConfigUL-BandwidthSRSBandwidthSub-frame configurationFrequency domain positionCyclic shiftDurationTransmission combConfiguration indexMaxUpPts
Measurements for UL positioning and UTDOA are performed on UL transmissions, which may include, for example, reference signal transmissions or data channel transmissions. UL RTOA is the currently standardized UTDOA timing measurement, and may be performed on Sounding Reference Signals (SRS). The results of the measurements are signaled by the measuring node (e.g., LMU) to the positioning node (e.g., E-SMLC), e.g., over SLmAP.
FIG. 2 illustrates the current architecture under discussion in 3GPP for downlink (DL) positioning, again including nodes found in the Radio Access Network (RAN) and the core network, as well as an external LCS Client. It will be appreciated that this architecture includes many of the same components found in the UL positioning architecture illustrated in FIG. 1. Two additional components shown in FIG. 2, however, are the Serving Gateway (S-GVV) and the Packet Data Network Gateway (PDN GW, or P-GW). These gateways terminate the UE's interfaces towards the E-UTRAN network and the Packet Data Network (PDN), respectively.
LPP is currently used for downlink positioning. An LPP message may also include an LPP extension packet data unit (EPDU); Open Mobile Alliance (OMA) LPP Extensions, defined as LPPe, take advantage of this possibility. Currently, LPP and LPPe are used mainly for downlink positioning, while LPPa may be used both for DL and UL positioning.
Positioning Results
A positioning result is a result of processing of obtained measurements, including Cell IDs, power levels, received radio signal strengths or quality, etc. The positioning result is often based on radio measurements (e.g., timing measurements such as timing advance and RTT, or power-based measurements such as received signal strength, or direction measurements such as angle-of-arrival measurements) received from measuring radio nodes (e.g., UE or eNodeB or LMU).
The positioning result may be exchanged among nodes in one of several pre-defined formats. The signaled positioning result is represented in a pre-defined format, e.g., corresponding to one of the seven Universal Geographical Area Description (GAD) shapes. Currently, a positioning result may be signaled between:                an LCS target, e.g., a UE, and an LCS server, e.g., over LPP protocol;        two positioning nodes, e.g., an E-SMLC or SLP, e.g., over a proprietary interface;        a positioning server (such as an E-SMLC,) and other network nodes, e.g., a Mobility Management Entity (MME), a Mobile Switching Center (MSC), a Gateway Mobile Location Center (GMLC), an Operations and Maintenance (O&M) node, a Self-Organizing Network (SON) node, and/or a Minimization of Drive Tests (MDT) node;        a positioning node and an LCS Client, e.g., between an E-SMLC and a Public Safety Access Point (PSAP), or between an SLP and an External LCS Client, or between an E-SMLC and a UE.Note that in emergency positioning, the LCS Client may reside in a PSAP.Uplink Positioning Measurements        
As the name suggests, measurements for uplink positioning (e.g., UTDOA) are performed on uplink transmissions, which may comprise, e.g., one or more of physical signal or channel transmissions, e.g., reference signal transmissions, random access channel transmissions, Physical Uplink Control Channel (PUCCH) transmissions, or data channel transmissions. Some examples of reference signals transmitted in LTE UL are SRS and demodulation reference signals.
UL Relative Time of Arrival (RTOA) is a currently standardized UTDOA timing measurement. The measurement may be performed on Sounding Reference Signals (SRS), which may be configured for periodic transmissions, typically comprising multiple transmissions but may also be one transmission. SRS transmissions may be triggered by any of the two trigger types:                Trigger type 0: higher layer signaling from eNodeB,        Trigger type 1: via downlink control channel signaling (DCI formats 0/4/1A for FDD and TDD and DCI formats 2B/2C for TDD).        
Other example uplink measurements are the uplink measurements specified in 3GPP TS 36.214. These measurements include measurements of received signal strength, received signal quality, angle-of-arrival (AoA), eNodeB receive-to-transmit (Rx-Tx) timing, relative time-of-arrival (RTOA), and other measurements performed by radio network nodes (e.g., eNodeB or LMU). Other known measurements are UL TDOA, UL TOA, UL propagation delay, etc.
Timing Measurements
In LTE, the following timing measurements are standardized in release 9:                UE Receive-Transmit (Rx-Tx) time difference,        eNodeB Rx-Tx time difference,        Timing advance (TA),        Reference Signal Time Difference (RSTD),        UE GNSS Timing of Cell Frames for UE positioning,        E-UTRAN GNSS Timing of Cell Frames for UE positioning.In the above list, the first, second, and third items are timing related measurements that are similar to round-trip-time (RTT) measurements used in earlier systems. These measurements are based on both downlink and uplink transmissions. In particular, for UE Rx-Tx, the UE measures the difference between the time of an uplink transmission and the time of the received downlink transmission that occurs after the UE uplink transmission. Similarly, for eNodeB Rx-Tx, the eNodeB measures the difference between the time of a downlink transmission and the time of the received uplink transmission that occurs after the eNodeB downlink transmission.        
In LTE there are additional timing measurements that are implementation dependent and not explicitly standardized. One example is a one-way propagation delay measurement. This is measured by eNodeB for estimation of a timing advance value to be signaled to the UE. Also, as stated above the UL RTOA measurement is being standardized for UTDOA.
The definitions of the various timing measurements in LTE are given below, as taken from the most recent version of the 3GPP specification 3GPP TS 36. 214:
For UE Rx-Tx time difference:
DefinitionThe UE Rx − Tx time difference is defined asTUE-RX − TUE-TXWhere:TUE-RX is the UE received timing of downlink radioframe #i from the serving cell, defined by the firstdetected path in time.TUE-TX is the UE transmit timing of uplinkradio frame #i.The reference point for the UE Rx − Tx time differencemeasurement shall be the UE antenna connector.Applicable forRRC_CONNECTED intra-frequencyFor eNodeB Rx-Tx time difference:
DefinitionThe eNB Rx − Tx time difference is defined asTeNB-RX − TeNB-TXWhere:TeNB-RX is the eNB received timing of uplink radioframe #i, defined by the first detected path in time.The reference point for TeNB-RX shall be the Rxantenna connector.TeNB-TX is the eNB transmit timing of downlinkradio frame #i.The reference point for TeNB-TX shall be the Txantenna connector.Timing advance measurement (TADV):
DefinitionType1:Timing advance (TADV) type 1 is defined as the timedifference:TADV = (eNB Rx − Tx time difference) +(UE Rx − Tx time difference),where the eNB Rx − Tx time difference corresponds tothe same UE that reports the UE Rx − Tx time difference.Type2:Timing advance (TADV) type 2 is defined as the timedifferenceTADV = (eNB Rx − Tx time difference),where the eNB Rx − Tx time difference corresponds toa received uplink radio frame containing PRACH fromthe respective UE.
Timing measurements may be used for positioning (e.g., with E-CID, AECID, pattern matching, hybrid positioning methods), network planning, SON, eICIC, in managing heterogeneous networks (e.g., for optimizing the cell ranges of different cell types), configuration of handover parameters, time coordinated scheduling, etc. Timing advance is also used to control the timing adjustment of UE UL transmissions as described later.
Multi-Carrier or Carrier Aggregation
To enhance peak rates within a technology, so-called multi-carrier or carrier aggregation solutions are known. Each carrier in multi-carrier or carrier aggregation system is generally termed as a component carrier, or sometimes referred to as a cell. In simple terms, the component carrier is an individual carrier in a multi-carrier system. The term carrier aggregation is also referred to with the terms (e.g., interchangeably called) “multi-carrier system”, “multi-cell operation”, “multi-carrier operation”, “multi-carrier” transmission and/or reception. Carrier aggregation is used for transmission of signaling and data in the uplink and downlink directions. One of the component carriers is the primary component carrier (PCC) or simply primary carrier or even anchor carrier. The remaining ones are called secondary component carriers (SCCs) or simply secondary carriers or even supplementary carriers. Generally the primary or anchor component carrier carries the essential UE specific signaling. The primary component carrier exists in both uplink and downlink direction in carrier aggregation. The network may assign different primary carriers to different UEs operating in the same sector or cell.
With carrier aggregation, the UE has more than one serving cell in downlink and/or in the uplink: one primary serving cell and one or more secondary serving cells operating on the PCC and SCCs respectively. The serving cell is interchangeably called the primary cell (PCell) or primary serving cell (PSC). Similarly, the secondary serving cell is interchangeably called the secondary cell (SCell) or secondary serving cell (SSC). Regardless of the terminology, the PCell and SCell(s) enable the UE to receive and/or transmit data. More specifically the PCell and SCell exist in downlink and uplink for the reception and transmission of data by the UE. The remaining non-serving cells are called neighbor cells.
Component carriers belonging to the CA may belong to the same frequency band (intra-band carrier aggregation) or to different frequency bands (inter-band carrier aggregation) or any combination thereof (e.g., two component carriers in band A and one component carrier in band B). Furthermore, the component carriers in intra-band carrier aggregation may be adjacent or non-adjacent in the frequency domain (intra-band, non-adjacent carrier aggregation). A hybrid carrier aggregation comprising any two of intra-band adjacent, intra-band non-adjacent and inter-band aggregations is also possible. Using carrier aggregation between carriers of different technologies is also referred to as “multi-RAT carrier aggregation” or “multi-RAT-multi-carrier system” or simply “inter-RAT carrier aggregation”. For example, carriers from WCDMA and LTE may be aggregated. Another example is the aggregation of carriers from LTE Frequency-Division Duplex (FDD) and LTE Time-Division Duplexing (TDD) modes, which may also be interchangeably called as multi-duplex carrier aggregation system. Yet another example is the aggregation of LTE and CDMA2000 carriers. For the sake of clarity, carrier aggregation within the same technology as described can be regarded as ‘intra-RAT’ or simply ‘single RAT’ carrier aggregation.
The component carriers in carrier aggregation may or may not be co-located in the same site or radio network node (e.g., a radio base station, relay, mobile relay, etc.). For instance, the component carriers may originate at different locations (e.g., from non-co-located base stations, or from base stations and a remote radio head (RRH), or at remote radio units (RRUs)). Well-known examples of combined carrier aggregation and multi-point communication techniques include the Distributed Antenna System (DAS), the Remote Radio Head (RRH), the Remote Radio Unit (RRU), and Coordinated Multipoint (CoMP) transmission. The techniques described herein also apply to multi-point carrier aggregation systems as well as to multi-point systems without carrier aggregation. The multi-carrier operation may also be used in conjunction with multi-antenna transmission. For example signals on each component carrier may be transmitted by the eNodeB to the UE over two or more antennas.
Uplink Transmit Timing Adjustment
In GSM systems, the mobile station (MS) sends its data three time slots after it receives the data from the base transceiver station (BTS). This approach works well as long as the distance between the MS and the BTS is small. However, increasing distances require consideration of propagation delay as well. To handle this issue, a Timing advance (TA) is conveyed by the network to the MS. A current value for the TA is sent to the MS within the layer-1 header of each Slow Associated Control Channel (SACCH) message. The BTS calculates the first TA for a given MS when it receives a Random Access Channel (RACH) from the MS, and reports the TA to the base station controller (BSC). The BSC/BTS pass the TA value to the MS during the Immediate Assignment procedure.
In UMTS Terrestrial Radio Access (UTRA) systems operating in Frequency-Division Duplexing (FDD) mode, the Timing Advance parameter is not used. Instead the network adjusts downlink timing to each UE, and thus implicitly adjusts the uplink timing, which is relative to the downlink timing.
More specifically, during a connection the UE may adjust the transmission time instant for a transmission on the Dedicated Physical Data Channel (DPDCH) and/or Dedicated Physical Control Channel (DPCCH). When the UE autonomously adjusts its DPDCH/DPCCH transmission time instant, it shall simultaneously adjust the transmission time instant for the High-Speed Dedicated Physical Control Channel (HS-DPCCH), the E-DCH Dedicated Physical Control Channel (E-DPCCH), E-DCH Dedicated Physical Data Channel (E-DPDCH) and secondary Dedicated Physical Control Channel (S-DPCCH) by the same amount, so that the relative timing between DPCCH/DPDCH and HS-DPCCH is kept constant and so that DPCCH/DPDCH and E-DPCCH/E-DPDCH and S-DPCCH remain time aligned. If the receive timing for any downlink DPCCH/DPDCH or F-DPCH in the current active set has drifted, so that the time between reception of the downlink DPCCH/DPDCH in question and transmission of uplink DPCCH/DPDCH lies outside the valid range, L1 informs higher layers, so that the network can be informed and so that downlink timing can be adjusted by the network. The maximum rate of uplink transmit time adjustment, and the valid range for the time between downlink DPCCH/DPDCH or F-DPCH reception and uplink DPCCH/DPDCH transmission in the UE are defined by system requirements.
In Time-Division Duplexing (TDD) mode, the UTRAN may adjust the UE transmission timing with timing advance. The initial value for timing advance (TAphys) is determined in the UTRAN by measurement of the timing of the Physical Random Access Channel (PRACH) or the E-DCH Random-Access Uplink Control Channel (E-RUCCH). The required timing advance is represented as an 8-bit number (0-255), referred to in the specifications as “UL Timing Advance′” or “TAul,” the UL Timing Advance being the multiplier of 4 chips that is nearest to the required timing advance (i.e., TAphys=TAul×4 chips). When Timing Advance is used, the UTRAN continuously measures the timing of a transmission from the UE and sends the necessary timing advance value. On receipt of this value, the UE adjusts the timing of its transmissions accordingly, in steps of ±4 chips.
The transmission of TA values is done by means of higher layer messages. Upon receiving the TA command the UE adjusts its transmission timing according to the timing advance command at the frame number specified by higher layer signaling. The UE is signaled the TA value in advance of the specified frame activation time, to allow for local processing of the command and application of the TA adjustment on the specified frame. The Node-B is also signaled the TA value and radio frame number that the TA adjustment is expected to take place.
If TA is enabled by higher layers, after handover the UE shall transmit in the new cell with a timing advance TA adjusted by the relative timing difference Δt between the new and the old cell:TAnew=TAold2Δt. 
In LTE systems, timing advance is generally handled as illustrated in FIG. 3, which illustrates the timing relationship between uplink and downlink subframes in LTE. According to 3GPP TS 26.211, transmission of the uplink radio frame number i from the UE shall start (NTA+NTA offset)×Ts seconds before the start of the corresponding downlink radio frame at the UE, where 0≦NTA≦20512, NTA offset=0 for frame structure type 1 (FDD and HD-FDD) and NTA offset 624 for frame structure type 2 (TDD). Note that not all slots in a radio frame may be transmitted. One example is TDD, where only a subset of the slots in a radio frame is transmitted.
When a UE wishes to establish a Radio Resource Control (RRC) connection with the eNodeB, it transmits a Random Access Preamble, and the eNodeB estimates the transmission timing of the terminal based on this. Then, the eNodeB transmits a Random Access Response, which consists of timing advance command, based on which the UE adjusts the terminal transmit timing. Timing advance commands may be sent later, as well, to maintain the UE uplink timing. The timing advance is initiated from E-UTRAN with a MAC message that implies an adjustment of the timing advance. The currently standardized Timing Advance Command MAC control element has a fixed size and consists of a single octet, as shown in FIG. 4. The first two bits, labeled “R” in FIG. 4, are reserved, and set to zero. The remaining 6 bits carry the Timing Advance Command. This field indicates the index value TA (0, 1, 2 . . . 63) used to control the amount of timing adjustment that UE has to apply.
To maintain uplink timing alignment, the UE uses a configurable timer timeAlignmentTimer to control how long the UE is considered to be uplink time-aligned. When a Timing Advance Command MAC control element is received, the UE applies the Timing Advance Command and (re)starts timeAlignmentTimer. If the timeAlignmentTimer expires, the UE flushes all Hybrid Automatic Repeat Request (HARQ) buffers, notifies RRC to release Physical Uplink Control Channel (PUCCH) and Sounding Reference Signal (SRS) resources, and clears any configured downlink assignments and uplink grants. According to the current standard, the UE shall not perform any uplink transmission except the Random Access Preamble transmission when timeAlignmentTimer is not running.
In LTE Release 8 and up to Release 10, there is only a single timing advance (TA) value per UE and for all uplink links, so a given UE has the same TA for transmission on all uplink carriers/cells if the UE supports uplink carrier aggregation. However, different UEs in a cell may have different transmit timings, and different UEs with uplink carrier aggregation transmitting in uplink carriers may have different timings. As of Release 11 of the 3GPP specifications for LTE, a UE on different carriers may have different timings, due to multiple TAs, as described below.
In LTE Release 11, support for multiple TA values was introduced, whereby a given UE may have different TA values for different serving cells. This is to support UE operation in a scenario in which the UE has different round trip delay with respect to different non-co-located physical nodes. Therefore the UE needs to transmit using different TA values to these non-co-located physical nodes. The most relevant scenario for the use of multiple TA values is the one in which the UE performs uplink transmissions to multiple uplink reception points. Each uplink transmission is sent on a different UL serving cell. For example a multi-carrier capable UE may transmit signals over carrier f1 and carrier f2 to an eNB and to a remote radio head (RRH) respectively, where the eNodeB and RRH are located at different sites. A UE might also need different TA values for uplink transmissions to cells in different bands, especially when the difference between the frequencies of different UL carriers used by the UE in multi-carrier is very large, e.g., where f1 and f2 operate on 700 MHz and 3500 MHz respectively.
There can be up to 5 downlink serving cells and 5 uplink serving cells according to 3GPP procedures in release 11. More than one serving cell can be grouped into the same TA group. The TA group containing the PCell is typically called the primary TA group, or pTAG. The pTAG may also contain SCell(s). The TA group containing only SCell(s) is typically called a secondary TA group, or sTAG. There is always only one pTAG, but there can be more than one sTAG. The pTAG and sTAG are UE-specific. Typically, cells with similar characteristics are grouped in the same TA group, but this depends on network implementation. For example all co-located cells are typically configured in the same TA group. Similarly, if possible, all cells in the same band that are co-located are also grouped in the same TA group. Information related to the TA groups is signaled by the network to the UE.
Serving cells in the same TA group share the same TA value. The downlink timing of one of the serving cells in the TA group is used by the UE as the timing reference for deriving its uplink transmit timing for autonomous uplink timing adjustment. (Autonomous uplink timing adjustments are discussed in detail below.) In a pTAG, the UE always uses PCell as the timing reference cell, for deriving the uplink transmit timing of the cells in the pTAG. In an sTAG, the UE uses one of the activated SCells as the timing reference cell for deriving the uplink transmit timing of the cells in the sTAG. If the timing reference SCell is deactivated, then the UE may select another activated SCell in that sTAG as a timing reference cell for deriving its uplink transmit timing.
Specific TA requirements in LTE include:                Timing Advance adjustment delay—the UE shall adjust the timing of its uplink transmission timing at sub-frame n+6 for a timing advancement command received in sub-frame n.        Timing Advance adjustment accuracy—the UE shall adjust the timing of its transmissions with an accuracy better than or equal to ±4*Ts seconds, relative to the signaled timing advance value compared to the timing of preceding uplink transmission. The timing advance command is expressed in multiples of 16*TS and is relative to the current uplink timing.Autonomous Uplink Timing Adjustment        
In addition to the TA based adjustment of the UL transmit timing described above, in the existing standards there is also pre-defined requirement on the UE to autonomously adjust its uplink timing in response to drifts in the eNodeB transmit timing. More specifically, the UE is required to follow the change in the frame transmit timing of the serving cell and correspondingly adjust its transmit timing for each transmission. The UE typically uses some sort of reference or pilot signals to track the downlink timing of the serving cell, such as the common reference signal, synchronization signals, etc.
A serving cell timing may change due to different reasons, such as due to variations in radio conditions, imperfection in clocks, maintenance activities, a deliberate attempt by the network to change timing or to compensate timing when timing drift exceeds a certain level, etc. In addition, it is also required that the UE changes its timing (increase or decrease) at a certain maximum rate. In other words, the size and number of adjustment steps applied over certain time is limited. The limit is ensured by the virtue of pre-determined requirements, which the UE should follow when the conditions are met, e.g., when UE transmit timing accuracy becomes worse than a pre-defined threshold. This is to make sure that the UE does not change the timing too fast. This requirement stems from the fact that if the UE changes its timing in the order of several microseconds (e.g., 3-4 microseconds) from one subframe to the next, the base station receiver may not be able to cope with the received signals. This will result in degradation of demodulation of signals transmitted by the UE and may ultimately result in uplink throughput loss.
Half-Duplex Operation
The LTE specifications enable Frequency Division Duplex (FDD) and Time Division Duplex (TDD) operation modes. Additionally, half-duplex operation is also specified, which is essentially FDD operation mode but with transmission and receptions not occurring simultaneously, as in TDD. FIG. 5 illustrates the basic differences between these three modes of operation. There is also another special case of an FDD band called “downlink FDD band” (also referred to as a “downlink FDD-only band”). A well-known example is that of LTE Downlink FDD band (716-728 MHz), which is being standardized. It does not have an uplink part of the spectrum and therefore for uplink transmission the downlink FDD band is always used in carrier aggregation mode with another FDD or TDD band. For example, downlink FDD band 712-728 might be used with LTE FDD band 2. TA for uplink on another band used with a downlink-only band would typically be sent by the downlink-only band.
Half-duplex mode has advantages with some frequency arrangements where the duplex filter may be unreasonable, which would result in high cost and high power consumption. Since the carrier frequency number (EARFCN) is unique, by knowing the carrier frequency number it is typically possible to determine the frequency band, which is either FDD or TDD, although the same part of the spectrum may be shared by multiple radio access technologies (RATs). Note that FDD and TDD are sometimes considered to be different RATs. Even if the same part of the spectrum is used for both FDD and TDD, their corresponding band numbers and other band-specific parameters (e.g., EARFCN) are different. A good example is that of 3.5 GHz spectrum. For instance, LTE FDD band #22 and LTE TDD band span the same part of the spectrum i.e. 3.4-3.6 GHz. However, it may be more difficult to find difference between full-duplex FDD and half-duplex FDD (HD-FDD) without explicit information, since a given FDD band can be used as full FDD or half-duplex-FDD (HD-FDD). HD-FDD is similar to FDD, but the UE cannot transmit and receive at the same time. HD-FDD is enforced by the eNodeB scheduler. The UE reports its capability to inform network whether or not it supports a particular FDD band in half duplex mode.
With respect to positioning operations, it should be noted that duplex information is currently not known to the LMU or positioning node.
Timing Advance with HD-FDD
In HD-FDD mode, from the UE perspective the uplink and downlink do not overlap in time. For DL-UL switching time, the UE ignores the end of the downlink subframe. For UL-DL switching time, some additional timing advance offset can be applied to the uplink transmissions. This is shown in FIG. 6, which illustrates the subframe timing relationship in HD-FDD mode.
Positioning Measurements for Different Duplex Modes
In LTE, OTDOA and E-CID positioning measurements and their corresponding requirements are specified for both FDD and TDD. For TDD mode, the same RSTD measurement definition, the same methods and the same accuracy requirements shall apply as for FDD. An exception is that for TDD the intra-frequency and inter-frequency RSTD measurement requirements are applicable only for selected uplink-downlink sub-frame configurations. In the case half-duplex FDD, the measurement period or reporting delay for certain positioning measurements may have to be extended or the requirements may have to be made applicable only to certain half-duplex configurations (i.e., when a particular number of downlink and/or uplink sub-frames are available).
In view of the introduction of new operating modes and carrier aggregation schemes in wireless communications systems, improved techniques for positioning are needed.